The pattern of reinnervation of the denervated canine heart has recently been described in terms of regional myocardial contractile force and regional myocardial catecholamine content. Functional reinnervation can be demonstrated to begin as early as 26 days and is complete between 9 and 18 months. Studies of myocardial catecholamine content indicate that the atria, especially the left, recover their norepinephrine content first followed by the basal ventricular areas finally by the apical areas. In animals studied as late as 26 months post denervation, norepinephrine levels are, however, only a small percentage of control values. Studies in subhuman primates have demonstrated a similar timetable for reinnervation. In man, no evidence of reinnervation has been detected as late as five years following cardiac transplantation. Recently, nerve growth factor (NGF) has been demonstrated to be a potent stimulator of growth of peripheral sympathetic sensory neurons. This factor is most effective on developing or growing neurons and when administered in vivo causes hypertrophy and hyperplasia of ganglia as well as increased innervation of viscera. It is proposed that NGF and NGF antisera be administered to dogs following cardiac denervation. Hemodynamic and chemical studies will be performed on these animals at specific intervals following denervation and the results compared to those obtained in the cardiac denervated, untreated, control animals. Data obtained from these studies will permit us to determine the influence of NGF on the rate and patterns of reinnervation of the canine heart. These data may also lead to an understanding of mechanisms involved in the failure of the human heart to reinnervate.